A method as described above is conventional. In a so-called common-rail fuel injection system, a fuel presupply pump delivers fuel from a fuel reservoir into a low-pressure region. A high-pressure pump then delivers the fuel from the low-pressure region into a high-pressure region. From there, the fuel is metered by controllable fuel injectors into the individual combustion chambers of the internal combustion engine. In conventional common-rail systems, the pressure in the high-pressure region, the so-called rail pressure, for gasoline direct-injection internal combustion engines is in a range between approximately 40 bar and several hundred bar, and for diesel direct-injection internal combustion engines reaches up to approximately 2000 bar.
A fuel mass introduced by the fuel injector into the combustion chamber is determined by an opening duration of the fuel injector and the rail pressure prevailing in the high-pressure region. During a start, particularly during a cold start, of the internal combustion engine, it is important that a precisely metered fuel mass be introduced into the combustion chamber. However, during a start, the rail pressure is frequently not yet completely built up, which has an unfavorable effect on the mixture formation in the combustion chamber. This is especially the case when the high-pressure pump is driven mechanically by a rotation of the internal combustion engine. In such a case, attempt is made with the aid of an electrically driven fuel presupply pump to generate a rail pressure, typically between 1 and 10 bar, which permits injection of a sufficient fuel mass into the combustion chamber.
However, the pressure generated by the fuel presupply pump may vary strongly between individual start phases, for example, caused by immobilization times of different lengths and different ambient temperatures. At very low temperatures, a lower battery voltage may cause an electrically driven fuel presupply pump to operate less effectively. At high temperatures in the fuel system, e.g., after switching off a vehicle having a hot internal combustion engine, a delayed pressure buildup may take place due to a formation of steam in the fuel system. The air/fuel mixtures thus formed in the combustion chamber may not be ignitable, which can lead to starting problems.
An object of the present invention is to improve the mixture formation of the air/fuel mixture needed for a start.